Removal of heat from electronic circuitry has become one of the limiting factors in the design and performance of most computer systems and many other electronic devices. Power dissipation of a computer increases approximately as the operating frequency squared. Thus, doubling the clock speed, roughly doubling computer system performance, will require nearly four times the power dissipation. Current microprocessors contain over one hundred million transistors. As the number of transistors increases with each new generation of microprocessor chips, the power draw and heat load (or total amount of heat) to dissipate during processor operation continues to increase. At the same time, the increasing processor heat loads coupled with smaller microprocessor footprints are causing the heat flux (heat load per unit area) to increase at a rapid rate.
Conventional cooling technologies such as fans and heat sinks are unable to absorb and transfer the excess heat dissipated by these new microprocessors. Attempts to cool these next generation microprocessors using conventional techniques are resulting in larger fans and larger heat sinks. The fans cause undesirable noise problems while metallic heat sinks add mass to computers and mechanical stress to the microprocessors to which they are attached. These mechanical factors can cause damage to the microprocessor during system transportation and handling.
Pumped liquid cooling systems have been used to provide improved thermal performance over conventional cooling technologies such as fan heat sinks and heat pipe heat sinks. Such systems typically include a cold plate heat sink that is attached to the microprocessor or other circuit to be cooled, a liquid-to-air heat exchanger, a single phase liquid pump, and connection tubing. A thermally conductive liquid coolant is circulated through the system by the pump. As the liquid passes through the cold plate heat sink, heat is transferred from the hot processor to the cooler liquid. The cold plate is designed in an attempt to maximize this heat transfer by maximizing the surface area to contact the cooling fluid flowing therethrough. For example, micro-channel cold plates utilize very fine fin channels through which the cooling fluid flows. Other cold plates employ powdered metal technology. Such cold plates use the high surface area density of bonded copper particles to enhance the effective heat transfer coefficient for heat dissipation. The hot liquid from the cold plate is then circulated through the liquid-to-air heat exchanger, that can be located at the rear of an electronic system enclosure or external to the enclosure. A system dedicated fan can be used to force air through the heat exchanger resulting in the transfer of heat to ambient air outside of the system enclosure. The cooled liquid then travels back through the system to repeat the process. Use of such closed liquid cooling systems is beginning to migrate from high performance computers to personal computers. It appears that liquid cooling systems will become more widely accepted and necessary in the future. However, even the best of such sealed pumped liquid cooling systems are limited in their ability to maintain low device temperatures without the use of refrigeration, and will not be able to satisfy the cooling demands of the next generation systems. This is because cold plates must be bonded to the outside of a chip package. Even a small amount of thermal resistance at the interface between the chip and the cold plate will create a large temperature difference between the cold plate and the chip. Without further innovation in the area of heat removal systems the development of a next generation computer design will be hampered.
One of very few practical ways to remove heat generated in the processing modules of very high-speed high-performance computers (supercomputers) is by forming sprays of small droplets that impinge on the heated surface forming a thin film of high velocity liquid directly on the computer chips. Heat is then transported from the surface by heating the flowing liquid and by boiling off some fraction of the liquid (two phase cooling). High heat removal rates are achieved because the liquid film is very well mixed by impinging spray droplets and bubbles formed by boiling. This mixing continually replenishes the supply of cool fluid at the heated surface. The limiting heat flux is reached when the liquid warms to its boiling point at any location on the surface being cooled. At this point the surface will dry out, the surface temperature will rise very quickly, and the device may be damaged. This method of heat removal is known as spray cooling, or spray evaporative cooling, and is a very efficient method of removing very high heat fluxes from small surfaces. Thus, spray evaporative cooling is growing in prominence and application in the cooling of electronics and laser diodes because of the need for high heat transfer in a small area in such applications.
The physical mechanisms of spray evaporative cooling are not completely understood. However, current research based on single nozzle spray systems suggests that less than 50% of the heat is removed through nucleate boiling, similar to boiling a pot of water on a stove. More than 50% of the heat is removed either directly by heating the flowing liquid film or by evaporation at the surface of the film. It also has been shown that the heat flux increases both with the area of the heated surface directly covered by the spray and by the flux of spray droplets impinging on the surface.
Current spray cooling systems and methods operate by spraying a dielectric fluid, e.g., Fluorinert 72 (FC-72), at a normal or other angle directly onto computer chips, from either above or below the chips, using a cone shaped spray. The objective is to create a thin film of cooling fluid on the surface to be cooled to remove heat through single phase and two phase convection. While such systems are adequate for most current computer systems and other electronic applications, the ability of such systems to remove heat has been pushed to its limits. For example, in such systems multiple nozzles often are used to direct multiple cone shaped cooling fluid sprays onto a surface to be cooled to increase the spray coverage area without increasing the package volume. However, in such systems the plumes of fluid that are sprayed from the nozzles tend to interact to create pockets of relatively low cooling fluid momentum, where the fluid tends to build up and where boiling often occurs. During the cooling process, bubbles are generated that cause greatly enhanced mixing so that heat is transported efficiently from the heated surface to the liquid surface. The temperature of the liquid surface is raised to the point that a large amount of evaporation occurs there, which is a safe and effective way to transport the heat away from the surface. Stagnation regions will stop the mechanical mixing, leading to bubble growth and dry out at the surface being cooled. Thus, the interaction of the sprays leads to inefficient cooling and liquid build-ups on the heated surface, creating regions of poor heat transfer and, therefore, non-uniform heat transfer across the surface of the chip. In the regions of poor heat transfer, the surface temperatures can rise significantly above the average surface temperature, causing the surface temperature to “run away”, leading to catastrophic failure.
For optimal operation, it also is imperative that the coolant be removed from the spray region quickly and efficiently to prevent the buildup of warm liquid. Failures also can occur if the coolant liquid is not efficiently removed from the surface being cooled. Current spray cooling systems are limited in the ability to drain spray coolant after impacting a heated surface. In particular, in many such systems changing the orientation of the system even slightly will affect coolant drainage patterns, thereby adversely affecting the cooling ability of the system. This limits the application of current spray cooling systems in many applications where the system to be cooled is portable and thus subject to changes in orientation.
Thus, current spray cooling systems suffer from various limitations including low critical heat flux (burnout) conditions, non-uniform heat transfer over large areas (area coverage limitations), and orientation constraints due to draining patterns.
A partial solution to many of these limitations of conventional spray cooling systems is described in U.S. patent application Ser. No. 11/070,683, filed on Mar. 2, 2005, by the inventors of the present invention and entitled FULL COVERAGE SPRAY AND DRAINAGE SYSTEM AND METHOD FOR ORIENTATION-INDEPENDENT REMOVAL OF HIGH HEAT FLUX. This application describes a spray cooling system and method that significantly improves spray evaporative cooling by creating a directed momentum flow of cooling fluid across a surface to be cooled. A spray of cooling fluid is directed directly onto the surface of a work piece to be cooled at an angle with respect to the work piece surface so as to create a flow of cooling fluid in a substantially single direction along the work piece surface. The spray of cooling fluid may be delivered, for example, via a plurality of generally fan shaped sprays. The sprays are positioned and aligned to create cooling fluid coverage of the entire heated surface to be cooled without allowing interaction between the spray plumes in a manner that may cause areas of cooling fluid stagnation on the surface.
Further improvement in spray cooling effectiveness and efficiency may, nevertheless, be obtained. For example, research has found the highest heat removal rates are achieved through single phase (non-boiling) heat transfer, because the surface to be cooled remains in contact with the cooling liquid. In this case, however, significantly more cooling fluid is required to transport heat away from the surface being cooled than in a two phase cooling system where boiling may occur. This is because in two phase, or phase change systems, energy removed from the surface is stored by breaking the inter-molecular bonds holding the liquid molecules together. This process can absorb a very large amount of energy per unit mass of coolant. On the other hand, single phase cooling relies on sensible heating of the liquid, or increasing the temperature of the liquid, to store the energy removed from the surface. Thus, to take advantage of the cooling effectiveness of single phase cooling systems more cooling fluid is required along with bigger pumps and other mechanical systems to transport the greater amount of fluid through the system. This thus increases the cost, weight, and power consumption of the system. In addition, the temperature of the single-phase liquid will rise as heat is added, causing potentially large temperature variations across the surface being cooled, which may be quite undesirable.
What is desired, therefore, is an improved evaporative spray cooling system and method that can achieve improved cooling capacity using less cooling fluid volume, that provides more uniform cooling over a surface being cooled, and that reduces the likelihood of critical heat flux.